net: microchip: sparx5: fix kconfig warning

PHY_SPARX5_SERDES depends on OF so SPARX5_SWITCH should also depend
on OF since 'select' does not follow any dependencies.

WARNING: unmet direct dependencies detected for PHY_SPARX5_SERDES
  Depends on [n]: (ARCH_SPARX5 || COMPILE_TEST [=n]) && OF [=n] && HAS_IOMEM [=y]
  Selected by [y]:
  - SPARX5_SWITCH [=y] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_MICROCHIP [=y] && NET_SWITCHDEV [=y] && HAS_IOMEM [=y]

Fixes: 3cfa11ba ("net: sparx5: add the basic sparx5 driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Lars Povlsen <lars.povlsen@microchip.com>
Cc: Steen Hegelund <Steen.Hegelund@microchip.com>
Cc: UNGLinuxDriver@microchip.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>